The focus of this project is to investigate the mechanisms whereby genetic information is transmitted to progeny somatic cells with fidelity: how mutagenesis occurs, and what mechanisms the cell employs to avoid mutation. Using a combination of classical genetic and recombinant DNA techniques, we have constructed a model system to examine the molecular basis of mutagenesis in the yeast, Saccharomyces cerevisiae. Construction of a yeast tester strain is nearing completion which will enable the mutagenesis of a cloned SUP4 tRNA suppressor gene to be assayed by direct genetic selection and DNA sequence analysis. In addition, the tester strain will allow the role in mutagenesis of the three genetically defined repair pathways of yeast to be examined using the same target plasmid.